Dense carbide composite bodies and method of making same

ABSTRACT

Hard, dense, composite ceramic bodies of boron carbide, silicon carbide and silicon, particularly useful as ceramic armor, are produced by forming a mixture of granular boron carbide and a temporary binder into a desired shape and setting the binder to obtain a coherent green body which is siliconized by heating it, in an inert atmosphere and in contact with a controlled amount of silicon, to a temperature above the melting point of silicon and in the range of about 1500-2200*C, whereupon the molten silicon infiltrates the body and reacts with some of the boron carbide thereof.

United States Patent 1191 Taylor et al.

1111 3,796,564 1451 Mar. 12', 1974 DENSE CARBIDE COMPOSITE BODIES ANDMETHOD OF MAKING SAME [75] Inventors: Kenneth M. Taylor, Niagara Falls;

Richard J. Palicka, Youngstown, both of N.Y.

[73] Assignee: The Carborundum Company,

Niagara Falls, NY.

[22] Filed: June 19, 1969 v 21 Appl. No.: 848,131

Related US. Application Data [62] Division of Ser. No. 640,327, May 22,1967.

Bartlett 106/44 X 3,725,015 4/1973 Weaver 75/202 X 3,730,826 5/1973Matchen et a1. 75/204 X 2,141,617 12/1938 Ridgway 106/43 2,529,33311/1950 Finlay 106/43 X 3,443,935 5/1969 Lipp 106/43 X I PrimaryExaminer-Carl D. Quarforth Assistant Examiner-P. A. Nelson Attorney,Agent, or Firm'David E. Dougherty; Raymond W. Green 5 7] ABSTRACT Hard,dense, composite ceramic bodies of boron carbide, silicon carbide andsilicon, particularly useful as ceramic armor, are produced by forming amixture of granular boron carbide and a temporary binder into a desiredshape and setting the binder to obtain a coherent green body which issiliconized by heating it, in an inert atmosphere and in contact with acontrolled amount of silicon, to a temperature above the melting pointof silicon and in the range of about 1500- 2200C, whereupon the moltensilicon infiltrates the body and reacts with some of the boron carbidethereof. I

9 Claims, No Drawings DENSE CARBIDE COMPOSITE BODIES AND METHOD OFMAKING SAME This is a division of application Ser. No. 640,327, filedMay 22, 1967.

BACKGROUND OF THE INVENTION This invention relates to hard, dense,composite ceramic bodies of boron carbide, silicon carbide and silicon;to methods of making such bodies; and to such hard, dense boroncarbide-silicon carbide-silicon composite bodies which are particularlyuseful as armor.

There has been considerable effort devoted in recent years to thedevelopment of ceramic armor materials which are sufficiently strong toafford adequate protection against projectiles but which at the sametime are light enough to be useful in such applications as personnel andaircraft armor, where it is important that the weight of the amount ofarmor required for protection be at a minimum or at least withinpractical limits. It is also important that the processes employed in'making ceramic armor materials be adaptable to the production of armorin rather intricate monolithic shapes such as helmets, vest sections,leg armor sections and the like.

Several comparatively lightweight ceramic materials have been reportedas useful for armor: sintered alumina; self-bonded silicon carbide; andhot pressed boron carbide. Of these, the alumina and silicon carbidematerials may be made by processes which permit the formation of variousshapes, but they have relatively high specific gravities of the order of3.65 and 3.05, respectively, and effective amounts of these materialsare comparatively heavy. Hot pressed boron carbide, on the other hand,has a relatively low specific gravity of about 2.45-2.5, but the natureof a hot pressing process is such as to render it difficult, if notimpossible, to form shapes other than flat plates and other relativelysimple shapes. Moreover, hot pressing is a particularly expensiveprocess and is not well suited to large scale production by continuousprocessing.

SUMMARY OF THE INVENTION The present invention contemplates a hard,dense (i.e., essentially nonporous) composite ceramic materialconsisting essentially of boron carbide, silicon carbide and silicon,which has a relatively low specific gravity approaching that of hotpressed boron carbide, but which may be produced without resort to hotpressing by an economical method which lends itself readilyto theproduction of bodies of various shapes. Accordingly, composite bodiesmay be produced in accordance with this invention which are especiallyuseful as ceramic armor. Such composite bodies may be quite suitable asarmor for protection against low caliber, low velocity projectiles, evenif they lack the optimum properties required for protection against highcaliber, high velocity projectiles. Composite bodies may also beprepared which, even if not especially useful as armor, nonetheless havedesirable properties such as hardness, wear resistance, strength andlack of porosity which render such bodies, in suitable shapes, useful asextrusion dies, sandblast nozzles, suction box covers for pa- Iper-making machines, buoyancy spheres and the like.

Briefly, the process of the invention comprises preparing an initialmixture of granular boron carbide and a temporary binder; forming theinitial mixture into a desired shape by pressing, extruding, investmentor slip casting or any other suitable method; setting the temporarybinder to impart sufficient coherence to the shaped green body to permitfurther processing; and siliconizing the coherent green body by heatingit, in an inert atmosphere which preferably is a vacuum and in contactwith a controlled amount of silicon, to a siliconizing temperature abovethe melting point of silicon and at least about l,500C but below about2,200C. Thereupon, the molten silicon infiltrates the body and undergoesa rather complex reaction with some of the boron carbide, producing somesilicon carbide in situ; and to the extent that interstices exist in thebody between the remaining boron carbide and the newly formed siliconcarbide, the interstitial space is permeated by free silicon.

DESCRIPTION OF PREFERRED EMBODIMENTS The invention will now be describedin detail and certain preferred features will be pointed out, partlywith reference to the examples whichfollow and which are intended toillustrate andnot to limit the inventive concepts.

EXAMPLE I A mixture of granular boron carbide of various grit sizes isprepared consisting of 45 parts of 180 grit, 25

parts of 400 grit and 30 parts of 800 grit. Thereto is mold, placed inan oven at room temperature and heated to 105C over a period of about 30hours at a substantially constant rate of temperature increase toevaporate the water and set the polyvinyl alcohol binder. The coherentgreen body thus obtained has a specific gravity of 1.55.

The green body is placed on a 7 inch square (17.7 cm square) piece ofloosely woven carbon cloth in the bottom of a dense graphite cruciblewhich is placed in a container disposed within the coils of an inductionfurnace. A 360 g quantity of granular silicon is carefully placed on topof the green body to form a pile of generally pyramidal shape with thebase of the pile extending near the edges of the body.

The container is evacuated to a pressure of about 50 microns to providean inert atmosphere, and the power source to the induction coils isturned on. The reduced pressure is maintained throughout the heatingcycle. An optical pyrometer sighted on 'the piece is used to ascertainthe temperature of the piece asthe temperature rises. At a temperatureof about 1,400C the silicon melts, spreading out over the top of thepiece; and when atemperature of about l,600C is reached, after a totalheating time of about 1.75 hours, the molten silicon infiltrates thepiece quite abruptly and reacts with some of the boron carbide toproduce silicon carbide. Thereupon, the power is immediately turned offand the furnace and its contents are allowed to-cool to roomtemperature.

The ceramic plate thus produced has a specific gravity of 2.54, amodulus of rupture of 25,000 psi (1750 kg/sq cm) and a modulus ofelasticity of 49 X psi (3.4 X 10 kg/sq cm), and is extremely hard andessentially nonporous. Anal.: total C, 14.18%; total Si, 41.79%; total B(diff.), 44.03%; free C, 0.07%; free Si, 2l,42%. X-ray diffractionanalysis indicates that the body consists of the following phases: afirst boron carbide type, with a diffraction pattern corresponding tonormal B C; a second boron carbide type, with a diffraction pattern ofboron carbide having an expanded lattice; alpha (hexagonal) siliconcarbide, beta (cubic) silicon carbide; and silicon.

The polyvinyl alcohol employed as the binder in Example 1 is dissipatedduring the siliconizing heating cycle, prior to the time that thesilicon melts, leaving no significant amount of carbon residue in thebody at the time the silicon infiltrates the body. For reasons whichwill be discussed hereinafter, however, it is often desired to have acertain amount of finely divided carbon present in the body at the timeof siliconization, in which case the silicon reacts not only with someof the boron carbide in the body, but with substantially all of thecarbon, thus producing silicon carbide from both of these carbonsources. One method of producing a body to be siliconized which containscarbon is to employ a temporary binder which is carbonizable, i.e.,which will produce a carbon residue in the body upon heating. Thismethod is illustrated in Examples 2 and 3. Altematively, a quantity offinely divided carbon of any suitable variety such as powdered graphitemay be incorporated in the initial mixture. It is usually preferred,however, when the presence of carbon is desired in the body to besiliconized, to adopt both of the foregoing methods by incorporatingcarbon and a carbonizable binder in the initial mixture, as in Examples4 and 5.

EXAMPLE 2 A mixture of granular boron carbide of various grit sizes isprepared consisting of 42 parts of 120 grit, 23 parts of 180 grit, 14parts of 400 grit and 14 parts of 800 grit. Thereto is added 7 parts ofa temporary carbonizable binder consisting of 53% of a liquidphenolformaldehyde resin of the kind typified by that sold by VarcumChemical Division of Reichhold Chemicals, Inc. under the trade nameVarcum 8121 and 47% of furfural as a diluent therefor. The mixture isblended until substantially homogeneous and screened through a coarsesieve to break up any agglomerates.

A 495 g quantity of the screened mix is pressed at 3,000 psi (210 kg/sqcm) to form a plate of the same dimensions as that prepared inExample 1. The piece is placed in an oven at room temperature and heatedto 150C over a period of about 24 hours at a substantially constant rateof temperature rise of about 5C/hr, to set and cure the resin, theresulting green body having a specific gravity of 1.58.

The body is then siliconized under substantially the same conditions andin the same manner as described in Example 1, employing 295 g of siliconand a vacuum of about 50 microns. As the temperature rises, but beforethe silicon melts, the binder carbonizes to produce carbon distributedthroughout the body, the amount of this residual carbon being about3540% of the weight of the resin incorporated in the initial mixture.Upon infiltration of the piece by the molten silicon, the silicon reactswith the residual carbon, as well as with some of the boron carbide, toproduce silicon carbide.

The ceramic plate, or tile, thus produced has a specific gravity of2,55, a modulus of rupture of 25,300 psi 1770 kg/sq cm), and a modulusof elasticity of 49.5 X 10 psi (3.5 X 10 kg/sq cm), and is extremelyhard and essentially nonporous. Anal.: total C, 14.0%; total Si, 32.2%;total B (diff.), 53.8%; free C, 0.18%; free Si, 21.0%.

EXAMPLE 3 A mixture of granular boron carbide is prepared consisting of18.7 parts of 16 grit, 23.4 parts of 36 grit, 1 1.7 parts of 54 grit, l1.7 parts of grit, 18.7 parts of 180 grit and 9.3 parts of 400 grit.Thereto is added 6.5 parts of the same carbonizable binder employed inExample 2 and the mixture is blended until substantially homogeneous andscreened through a coarse sieve to break up any agglomerates.Substantially the same procedure and conditions as in Example 2 areemployed to prepare and siliconize a green body from 520 of the mix, 300g of silicon being used. The resulting plate, of the same dimensions asthat prepared in Example 2, has a specific gravity of 2,51, a modulus ofrupture of 12,000 psi (840 kg/sq cm) and a modulus of elasticity of 44.5x 10 psi (3.1 X 10 kg/sq cm), and is extremely hard and essentiallynonporous. Anal.: total C, 15.2%; total Si, 35.3%; total B (diff.),49.5%; free C, 0.35%; free Si, 20.0%.

EXAMPLE 4 A mixture is prepared consisting of 45 parts of grit boroncarbide, 23 parts of 180 grit boron carbide, 30 parts of 800 grit boroncarbide, 2 parts of powdered graphite with a particle size of about 50microns and smaller, and 9 parts of a carbonizable binder consisting of46% of a phenol-formaldehyde resin such as that used in Example 2 and54% of furfural as a diluent therefor. The mixture is blended untilsubstantially homogeneous and screened through a coarse sieve to breakup any agglomerates. A 545 g quantity of the mix is pressed at 2,700 psito form a plate having the same dimensions as that prepared in Example1, and the plate is placed in an oven at room temperature and heated toC over a period of about 24 hours at a rate of temperature rise of about5C/hr, to set and cure the resin. The resulting green body has aspecific gravity of 1.73.

The piece is siliconized in accordance with the method and conditionsset forth in Example I, employing 282 g of silicon, the bindercarbonizing during the heating before the silicon melts. Uponinfiltration of the piece by the molten silicon, the silicon reacts withthe powdered graphite, the residual carbon from the binder, and some ofthe boron carbide, to produce silicon carbide.

The resulting plate has a specific gravity of 2,57, a modulus of ruptureof 34,700 psi (2,400 kg/sq cm) and a modulus of elasticity of 51.2 X 10psi (3.6 X 10 kg/sq cm), and is extremely hard and essentiallynonporous. Anal.: total C, 16.5%, total Si, 32.6%, total B (diff.),50.9%; free C, 0.16%; free Si, 12.6%.

EXAMPLE 5 A mixture is prepared consisting of 40 parts of 120 grit boroncarbide, 30 parts of grit boron carbide, 10 parts of 400 grit boroncarbide, 20 parts of powdered graphite with a particle size of about 50microns and smaller, and parts of the same carbonizable binder employedin Example 4. The mixture is blended until substantially homogeneous andscreened through a coarse sieve to break up any agglomerates. A 490 gquantity of the mix is pressed at 3000 psi to form a plate having thesame dimensions as that prepared in Example 1, and the plate is placedin an oven at room temperature and heated to 150C over a 24 hour periodat a rate of temperature rise of about 5C/hr to set and cure the resin.The resulting green body has a specific gravity of 1.55.

The piece is siliconized in accordance with the method and conditions ofExample 1, employing 320 g of silicon, the binder carbonizing during theheating.

The resulting plate has a specific gravity of 2.72, and is extremelyhard and essentially nonporous. Anal: total C, 22.5%; total Si, 39.1%;total B (diffi), 38.4%; free C, 0.71%; free Si, 6.9%.

Knoop 100 hardness determinations on the boron carbide, silicon carbideand silicon present in the bodies of this invention give values of about2850-3000, 2500-2700 and 700-1600 kg/mm sq, respectively, the value forsilicon being somewhat higher than the expected value of 700-900,possibly due to small amounts of boron being present in the silicon.Since the silicon phase is considerably softer than the boron carbideand silicon carbide, it is usually preferred to produce bodiescontaining the minimum amount of free silicon consistent with obtaininga sound, .uncracked body, when hard bodies suitable for use as armor aredesired. Several interrelated process variables must be considered inthis connection, including the particle size of the boron carbide in theinitial mixture and the presence of carbon in the green body beingsiliconized.

The particle size of the granular boron carbide is convenientlyexpressed as grit size, each grit size number designating theapproximate range of particle sizes set forth in Table I.

TABLE I Typical Particle Size Distribution of Boron Carbide AbrasiveGrits Dense boron carbide-silicon carbide-silicon bodies may be preparedby the method of the invention from an initial mixture containing boroncarbide grain of a single, uniform particle size. It is usuallydisadvantageous to do so, however, because green bodies formed withparticles of uniform size have considerably more interstitial spaceunoccupied by boron carbide than bodies formed with particles of varyingsizes, and therefore tend to have a comparatively high free siliconcontent after siliconization. It is therefore almost always preferred,and is essential for producing highly effective armor, to employ avariety of boron carbide grit sizes in preparing the initial mixture, asin Examples l-5, the variety being such as to permit dense packing andresult in a green body having comparatively little interstitial spacebetween the boron carbide particles.

A combination of about of relatively coarse grit and about 30% ofrelatively fine grit is usually suitable, at least as a starting pointfor further refinements which may be introduced on the basis ofexperimental determinations of the optimum combination for the intendedpurpose. Thus the granular boron carbide of Example 1 consists of 70%relatively coarse (180 and 400) grit and 30% relatively fine (800) grit;the granular boron carbide of Example 2 consists of 70% and grit and 30%400 and 800 grit; the granular boron carbide of Example 3 consists of70% 16, 36, 54 and 70 grit and 30% 180 and 400 grit; and the granularboron carbide of Example 4 consists of about 69% 120 and 180 grit andabout 31% 800 grit. It should be noted that the terms relatively coarseand relatively fine refer to the relationship between the particle sizespresent in a given initial mixture, and any numerically designated gritsize will be relatively coarse or relatively fine, depending upon theother grit sizes present.

From a standpoint of producing siliconized bodies of maximum strengthand hardness and which are highly effective as armor, it is essentialthat the granular boron carbide in the initial mixture have a maximumparticle size of about 300 microns or less, although coarser materialmay be employed to make composite bodies useful for less demandingpurposes. The maximum boron carbide particle size in Example 1. is about1 14 microns (180 grit) and most of the boron carbide is considerablyfiner, this mixture having the smallest maximum particle sizeillustrated by the examples; and plates produced as in Example 1 proveto be highly effective as armor when subjected to rigorous, controlledballistic tests which involve firing projectiles of selected sizes withcontrolled velocities at the center of the plates. From a practicalstandpoint, however, there is a limit of how fine the boron carbidegrain may be, since as the particle size of the boron carbide isdecreased its reactivity with the silicon during the siliconizationincreases, and it becomes increasingly difficult to control thesiliconization and prevent cracking of the bodies which occurs,apparently as a result of a rapid and extensive reaction. In fact, it isdifficult to produce sound, intact bodies consistently when using the180-400-800 grit mixture employed in Example 1. For this practicalreason, it is usually preferred to employ at least some granular boroncarbide which is coarser than 180 grit. While the use of such coarsermaterial tends to result in an increase in the volume of interstitialspace in the green body and therefore an increase in the amount of freesilicon in the siliconized body, this tendency toward an increase in theamount of free silicon can be at least partially offset by the presenceof carbon in the green body at the time it is siliconized, which may beachieved by the incorporation of carbon and/or a carbonizable binder inthe initial mixture.

Comparing Examples 1 and 2 in this light, it is seen that the freesilicon content of the bodies produced is substantially the same,notwithstanding the inclusion of some 120 grit boron carbide in theinitial mixture of Example 2, in which a carbonizable binder is alsoincorporated. As the green body of Example 2 is heated and the binder iscarbonized, carbon is produced in the interstices between the boroncarbide grains; and upon infiltration the molten silicon reacts with thecarbon, as well as with the boron carbide, to produce silicon carbide,thus reducing the volume of interstitial space available for occupancyby free silicon. The body produced in Example 2 is comparable to that ofExample 1 as armor in controlled ballistic tests, and the modulus ofrupture and modulus of elasticity of the bodies of Examples l and 2 areabout the same.

Comparing Example 2 with Example 3, in which a substantial amount ofboron carbide grain larger than 300 microns is used in the initialmixture, it may be seen that the modulus of rupture and modulus ofelasticity are markedly lower for the body produced in Example 3although the free silicon content of the bodies of both examples isabout the same, a carbonizable binder being employed in both examples.Ballistic tests on bodies prepared as in Example 3 show that they areclearly inferior to bodies prepared as in Example 2, although useful asarmor for protection against low velocity, low caliber projectiles.Accordingly, it is usually preferred to exclude from the initial mixtureany substantial amount of boron carbide with a particle size of morethan about 300 microns.

Example 4 illustrates a particularly preferred and highly desirableinitial mixture for the production of composite bodies according to theinvention. It will be noted that the maximum particle size of thegranular boron carbide is well below 300 microns, but that some of theboron carbide is coarser than 180 grit. In addition to employing acarbonizable binder to provide for the presence of carbon in the greenbody at the time the silicon infiltrates, carbon in the form of powderedgraphite is incorporated in the initial mixture to aid in minimizing theamount of free silicon in the body after siliconization. The compositebody produced in Example 4 contains considerably less free silicon thanthe body of Example 1, and has superior mechanical properties. Ballistictests show that the composite body of Example 4 is as good as or betterthan that of Example 1 for use as armor, and the initial mixture ofExample 4 is preferred because composite bodies produced therefrom are,by virtue of the incorporation of boron carbide larger than 180 grit inthe initial mixture, much less subject to cracking during or followingsiliconization. Yields of 8090% are attainable using the process ofExample 4; i.e., sound, crack-free siliconized bodies are obtained inabout eight or nine out of ten runs.

Considering Example 5, it is seen that by incorporating a comparativelylarge amount of carbon in the initial mixture, the amount of freesilicon in composite bodies of the invention may be reduced to a ratherlow level, 6.9% in this example. Bodies containing as little as about 3%free silicon can be produced by suitably adjusting the boron carbidegrit sizes and the amounts of carbon and carbonizable binder in theinitial mixture and optimizing the specific gravity of the coherentgreen body. It will also be noted that the specific gravity of thecomposite body produced in Example 5 is somehwat higher than thespecific gravity of bodies of higher free silicon content producedaccording to Examples l-4. This is to be expected, since silicon has arelatively low specific gravity of 2.33 as compared to boron carbide andsilicon carbide with specific gravities of 2.5 and 3.2, respectively.

It is difficult, however, to consistently produce sound composite bodieshaving such a low content of free silicon unless the bodies arecomparatively small, for reasons which are not entirely clear. Attemptsto repeat Example 5 often result in cracked bodies, and the problembecomes worse as the size of the body sought to be made is increased. Itmay be conjectured that the cracking is due, at least in part, tostresses developed as a result of the expansion which occurs when thecarbon reacts with silicon to form silicon carbide. In any event, itappears that about 3% free silicon is the approximate practicallyattainable minimum.

Even though composite bodies of such low silicon content may be producedand are highly effective as armor, the practical difiiculty of achievinggood yields makes it commercially preferable to produce bodiescontaining at least about 10% silicon. Bodies prepared according toExample 4, which describes a more or less optimum compromise in respectof the numerous variables involved in the process of the invention,usually contain from about 12% to about 16% free silicon.

Summarizing the foregoing, the initial mixture to be used in producingcomposite bodies of the invention may comprise granular boron carbide ofa single, uniform particle size but preferably contains boron carbidewith a variety of particle sizes, the variety preferably being such asto permit dense packing and leave a minimum of interstitial space whenthe initial mixture is formed into the desired shape. Further, it ispreferred that the boron carbide have a maximum particle size of about300 microns or less, and that it contain some grain which is coarserthan grit. The initial mixture advantageously contains a carbonizabletemporary binder or carbon, and preferably contains both.

The temporary binder may be selected from among a wide variety ofmaterials recognized as suitable for such use, for example, polyethyleneglycols and methoxypolyethylene glycols such as those sold by UnionCarbide Corporation under the trade name CARBO- WAX, polyvinyl alcohol,and where a carbonizable binder is desired, phenolformaldehyde resins,epoxy resins, dextrine and the like.

The binder is employed in an amount sufficient to give an initialmixture which is of the proper consistency for forming into the desiredshape by the method to be employed. Even if the binder is of thecarbonizable type this amount is usually established without regard tothe amount of carbon present therein, since finely divided carbon can beincorporated in the initial mixture in an amount sufficient to providethe total quantity of carbon desired in the green body at the time ofsiliconization. However, it is usually preferred that the carbonizablebinder have a high carbon content so that as much of the carbon in thebody as possible comes from this source, such carbon generally beingmore finely divided and dispersed than the carbon added to the mix. Thecarbon added to the mix may be graphite, such as employed in theexamples, or another type of free carbon.

For any given initial mixture of granular boron carbide and binder, theoptimum amount of carbon to produce a composite body having the desiredproperties is best determined experimentally. Calculations based uponthe comparative specific gravities of the unsiliconized and siliconizedbodies and the computed interstitial volumes thereof do not afford ameaningful guide because the silicon reacts not only with the carbon,but with the boron carbide as well, and the extent of the latterreaction and effect thereof is not precisely predictable. Generally, theoptimum amount of carbon is that which will give good yields ofsiliconized bodies which are sound and not subject to cracking, and ofminimum free silicon content consistent therewith. An excess of carbontends to result in cracked bodies, presumably due to the expansion whichaccompanies the conversion of carbon to silicon carbide, while aninsufficiency of carbon tends to result in a higher free silicon contentin the composite body. The graphite in the desirable initial mixture ofExample 4, present in an amount of about 2% of the boroncarbide-graphite portion of the mix, approaches the optimum amount forthe mix described, since an increase to 3% usually results in crackingduring siliconization, while a decrease tends to result in a higher freesilicon content and correspondingly inferior properties in the compositebody.

After the ingredients of the initial mixture have been blended togetherand, if necessary, passed through a coarse screen to break 'up anyagglomerates, the mixture is formed into the desired shape and thebinder is set, to produce a coherent green body.

Various methods may be employed to shape the mix. In general, themechanical properties of the composite bodies tend to improve as thespecific gravity of the green body is increased, and shaped mixes ofmaximum specific gravity and minimum interstitial space are thereforeusually preferred, subject, however, to the limitation that there mustbe a continuous interconnected network of interstitial space between theboron carbide grains throughout the green body to permit theinfiltration of silicon. In practice this limitation is of littlesignificance because it would require extreme and specialized measuresto attain such close packing and high specific gravity in the shaped mixas to present a problem of insufficient porosity. Even when the initialmixture is pressed into the shape of a plate at pressures up to 3,000psi as in the examples, the specific gravity of the green body is onlyabout 70% of the theoretical specific gravity, and the green bodies aresufficiently porous.

When employing the desirable initial mixture of Example 4 to form flatplates by pressing, it is usually preferred to use a pressure of about2,5003,000 psi, a green body having a specific gravity of about1.70-1.76 being obtained upon setting the binder. Pressures less thanabout 2,500 psi tend to result in a lower specific gravity, moreporosity in the green body, and a higher silicon content aftersiliconization, while pressures considerably in excess of 3,000 psiappear to give no further densification. When other initial mixtures areemployed, the optimum pressure varies to some extent with the particlesize of the boron carbide, somewhat less pressure being required toattain a given specific gravity as the particle size decreases. Theamount of initial mixture employed will, of course, depend upon thevolume and specific gravity of the shaped mix.

Although the examples illustrate shaping of the initial mixture only bypressing, various other methods such as extrusion, slip casting andinvestment casting may be used, and the method of choice will dependprimarily upon the shape desired. The composition of the initial mixturemay be varied, especially in respect of the binder, to obtain the mostsuitable mix for the particular method of forming employed.

For investment casting and slip casting a particularly desirable initialmixture is substantially the same as that described in Example 4 exceptfor the binder, and consists of 45 parts of 120 grit boron carbide, 23parts of 180 grit boron carbide, 30 parts of 800 grit boron carbide, 2parts of powdered graphite, and 50 parts of a 4% solution of dextrine inwater. Preferably a small quantity of a viscosity-lowering agent isincorporated to render the mix more flowable, for example, apolyelectrolyte such as the one sold by R. T. Vanderbilt Company underthe trade name DARVAN 7. Casting techniques are well-known andaccordingly not described here in detail. In investment casting a porousmold with a cavity of suitable configuration to produce the desiredshape is filled with the mix, preferably with vibration to aid packingof the particles. The filled mold is allowed to stand until sufficientaqueous phase has been absorbed from the mixture into the porous mold topermit the body to be handled. The body may then be removed from themold and the binder set, Using the above-mentioned mixture, coherentgreen bodies having a preferred specific gravity of about 1.7 may beobtained by investment casting in intricate shapes such as helmets, legarmor sections, vest sections, etc., which bodies may then be heated toCarbonize the binder and siliconized to produce composite bodiesaccording to the invention which may be used as armor or for numerousother purposes.

Another particularly useful embodiment of the invention involves formingthe initial mixture by extrusion. It is well-known that boron carbide isof limited utility as an abrasive because the grain tends to fractureconchoidally and thus become rounded and less abrasive as wear occursduring use. However, an initial mixture according to the presentinvention may be extruded to form a fine strand, which may be cut intoshort lengths and the binder therein set to produce grain which may besiliconized to obtain boron carbidesilicon carbide-silicon abrasivegrain. It has been found that such grain, even though containing asubstantial amount of boron carbide, nonetheless fractures in use insuch a way as to leave new, sharp, abrasive edges exposed, such graintherefore being well suited to use in abrasive articles.

When the mixture has been formed into the desired shape, the binder isset to obtain a coherent green body under conditions suited to theparticular binder employed. Usually a slow heating cycle such as used inthe examples is appropriate, the temperature being increased gradu'allyto permit dissipation of the volatiles without cracks forming in thebody. When a phenolformaldehyde or other resin is employed, setting thebinder may also involve curing the resin, whereas with solutions ofpolyvinyl alcohol, dextrine and the like, setting the binder primarilyinvolves evaporation of the solvent.

When a carbonizable binder is employed, the binder in the coherent greenbody may be carbonized by heating the body in an inert atmosphere toasufficiently high temperature to effect carbonization. Since volatilesare dissipated from the body during carbonization, it may be necessaryor desirable to control the carbonizing heating cycle by providing for aslow rate of temperature increase to permit the escape of volatileswithout cracking the body. The need for such control tends to increasewith increasing size and thickness of the body. As may be seen from theexamples, no specific temperature control is required under theconditions there set forth for bodies of the described dimensions; andwhile carbonization may, if desired, be carried out as a separate step,it may often conveniently be accomplished during the siliconizingheating cycle.

Siliconization of the green body is carried out by heating it, in aninert atmosphere and in contact with a controlled amount of silicon, toa siliconizing temperlil ature above the melting point of silicon and atleast about 1 ,500C, at which temperature the molten silicon infiltratesthe body and reacts with some of the boron carbide and withsubstantially all of the carbon present, if any, producing siliconcarbide in situ. The siliconization is conveniently carried out asdescribed in the examples, by placing the green body in a suitablecrucible in a suitable furnace with the requisite amount of granularsilicon spread on top of the body. A piece of carbon cloth, convenientlycarbonized burlap, may be placed between the crucible and the piece tobe siliconized to minimize any tendency for the piece to adhere to thecrucible.

The amount of silicon employed must be carefully controlled withinrather narrow limits to obtain sound, uncracked composite bodies havingoptimum properties for use as armor,'this control being necessary toprevent the reaction of boron carbide and silicon from proceeding to anundesirable extent. Since the tendency for this reaction to proceeddepends largely upon the particle size of the boron carbide, the degreeof control over the amount of silicon must, in general, be greater asthe boron carbide particle size is decreased. However, since it isusually preferred to employ at least some rather fine boron carbide inthe mix in order to produce bodies of superior properties, the amount ofsilicon used must usually be carefully regulated.

The amount of silicon to be used for a given green body ordinarilycannot be calculated with exactitude since it is difficult to predicthow much silicon will react with the boron carbide and how much will bepresent in the siliconized body as free silicon. However, a reasonablyclose approximation of the requisite amount of silicon can be made bysubtracting the measured specific gravity of the coherent green bodyfrom the approximate desired specific gravity of the siliconized bodyand multiplying by the volume of the body, thus computing the amount ofsilicon needed to give the desired weight increase, assuming as is truethat there is no appreciable change in the dimensions of the green bodyupon siliconization. If carbon cloth is employed, it will generallyreact stoichiometrically with the silicon to form silicon carbide, andthe additional amount of silicon required for this purpose can becalculated quite accurately. The precise optimum amount of silicon isbest determined experimentally for a green body of given dimensions andcomposition, using the calculated approximation as a starting pointwhich is subject to modification, to obtain a sound, essentiallynonporous composite body of the desired specific gravity. The body ispreferably free of any excess silicon on its surface, thus requiring nopolishing prior to use. Preferably there is no adhesion of the body tothe crucible, or to the carbon cloth if one is employed.

A small excess or deficiency of silicon can seriously affect the courseof the siliconization. Excess silicon gives rise to an increasedtendency of the body to crack, probably due at least in part to anunduly extensive reaction with the boron carbide, and possibly also dueto certain surface effects of the excess silicon resulting in anonuniform body having variable coefficients of thermal expansion. Adeficiency of silicon results in a body of greater porosity,disadvantageous for armor, and may result in a cracked body. In Example2, where 295 g of silicon is employed, it is found that sound, uncrackedbodies are produced only if the total amount of silicon is no less thanabout 270 g and no more than about 320 g.

It is desirable for the molten silicon to contact the piece as uniformlyas possible, since a substantial localized excess tends to result incracks at that location. Little difficulty is bad in siliconizing asubstantially flat piece, the granular silicon merely being placed ontop of the piece where, upon reaching the melting point, it melts andcovers the surface. Bodies of more complex shapes such as cylinders maybe wrapped with a porous wick material such as carbon cloth, placed inthe crucible on another piece of carbon cloth, and surrounded with theproper amount of granular silicon so that upon melting the silicon isdrawn up by the wick, contacts the green body quite uniformly, andinfiltrates the body when a sufficiently high temperature is reached.

Siliconization must be carried out in an inert atmosphere such as vacuumor an inert gas, e.g. argon, helium or the like. Nitrogen is notsuitable because it tends to react with the boron carbide at thetemperatures reached to form a coating of boron nitride on the piece,which coating impedes infiltration of the silicon. Vacuum is muchpreferred, and the higher the better, since it aids in the removal ofany air trapped within the piece. High vacuum permits infiltration ofthe silicon at temperatures as low as about 1,500C, and a temperature ofabout 1,600C is required at a pressure of about 50 microns. In anatmosphere such as argon or helium at atmospheric pressure, a somewhathigher siliconizing temperature of about 1,850C is usually required,this higher temperature being less desirable as a result of increasedreactivity of the boron carbide with concomitant greater difficulty incontrolling the siliconization to give a sound, uncracked compositebody. It is usually found that while inert atmospheres other than vacuumare suitable for smaller pieces, increasing difficulty is experienceddue to localized siliconization and cracking as the size of the body isincreased.

When heated to about 2,200C, it appears that a phase change occurs inthe body, accompanied by melting and deformation. Furthermore, moltensilicon will tend to evaporate rapidly under vacuum at temperatures wellbelow 2,200F. Thus 2,200C represents the highest practicable temperaturefor siliconization, and preferably the lowest possible temperature isused in order to minimize the rate of reaction of the boron carbide withthe silicon.

It is apparent that various suitable types of furnaces may be usedinstead of the induction furnace of the examples. The rate oftemperature rise below the melting point of silicon is not especiallyimportant except as regards carbonization of the binder as discussedabove. When the silicon has melted, however, it is desirable that thetemperature increase as rapidly as possible to that at whichinfiltration occurs, to minimize any effects which the molten siliconmight have on the surface'of the piece; and once infiltration hasoccurred, it is desirable to avoid any further temperature increase andto allow the piece to cool as rapidly as possible without causingcracking due to thermal shock to minimize the extent of the reactionbetween the boron carbide and silicon and thereby minimize the tendencyof the body to crack.

The composition and structure of the composite bodies of the inventionappear to be complex. X-ray diffraction analysis of the bodies producedin Examples 25 indicates that each body contains the same five phases asreported for the body of Example 1, and bodies produced in accordancewith the invention invariably appear to consist essentially of boroncarbide, silicon carbide and free silicon. It has been observed,however, that composite bodies siliconized at temperatures of about1,850C usually have x-ray diffraction patterns which indicate that thebodies contain no beta-silicon carbide, only the alpha form beingpresent, whereas bodies siliconized at about 1,600C usually appear tocontain both forms of silicon carbide. The two boron carbide typesinvariably appear to be present, both having a boron carbide typer'hornbohedral structure, one of which types corresponds to B C, theother having an expanded lattice and being of less determinatecomposition but containing at least boron and carbon and possiblycontaining some silicon. There is some indication that the free siliconmay, at least in some cases, contain minor amounts of boron. Minorimpurities, such as iron and calcium which may be present in thegranular silicon or boron carbide employed, may also be present in thecomposite body.

The silicon carbide and boron carbide are present as mutually-adherentgrains in the composite body, presumably as a result of the reactionwhereby silicon carbide is formed from the boron carbide. When freesilicon is present in amounts up to about it appears as a discontinuousphase, filling the interstices between the boron carbide and siliconcarbide; when present in amounts of about 15% or more, it appears as acontinuous phase which further bonds the boron carbide and siliconcarbide. It is usually found that the modulus of rupture is somewhatlower in bodies containing more than about 15% free silicon than inbodies containing less than about 15% thereof. As noted above, 3%silicon appears to be about the minimum attainable and amounts of atleast about 10% are preferred. While there is no specific upper limit tothe free silicon content of the composite'bodies, and bodies containingconsiderably more than 35% are easily produced, it is usually preferredthat the silicon content be no more than about if highly effective armoris desired. Bodies containing from about 10% to about 25% of freesilicon usually contain from about 50% to about 80% boron carbide andfrom about 10% to about 25% silicon carbide.

The composite bodies of the invention are essentially nonporous. Theirspecific gravity is generally within the range from about 2.5 to about2.75, but for armor it is usually preferred that the specific gravityrange from about 2.5 to about 2.6 since such composite bodies have theadvantage of being lighter in weight while equally effective as armor.The modulus of rupture may be as low as about 10,000 psi (700 kg/sq cm),especially when granular boron carbide with a particle size greater thanabout 300 microns is included in the mix, but may be as high as about37,000 psi (2600 kg/sq cm) or more and is preferably at least about20.000 psi (1400 kg/sq cm) for use as armor. The modulus of elasticityusually ranges from a low of about X 10 psi (2.] X 10 kg/sq cm),especially when granular boron carbide with a particle size greater thanabout 300 microns is included in the mix, to a high of about 60 X 10 psi(4.2 X 10 kg/sq cm), and preferably is at least about 45 X 10 psi(3.1 X10 kg/sq cm).

The composite bodies of the invention are virtually insoluble in diluteor concentrated hydrochloric, sulfuric or nitric acid, but may showslight weight loss in a hot 50% sodium hydroxide solution after 24hours. At l,350C, bodies prepared in accordance with Example 4 show goodoxidation resistance and only about 10% reduction of their modulus ofelasticity.

Except as otherwise spcified, all references herein to percentages andto parts refer, respectively, to percentages and parts by weight; andreferences to microns or millimeters in describing pressure and vacuumrefer to microns or millimeters of mercury. Except as otherwisespecified, modulus of rupture determinations are made by the four pointloading system, and modulus of elasticity determinations are made by thesonic method.

We claim:

1. A method of making a dense, composite ceramic body which comprisespreparing a substantially homogeneous initial mixture of granular boroncarbide and a temporary binder, forming said mixture into a desiredshape, setting the binder to obtain a coherent green body, and heatingsaid green body in either an inert atmosphere or a vacuum in thepresence of a controlled amount of granular silicon to a temperatureabove the melting point of silicon and at least about l,500C but belowabout 2,200C, whereby the silicon becomes molten, said granular siliconbeing so disposed externally to said green body that said molten siliconsubstantially uniformly contacts the surface of said green body andsubsequently permeates said green body, whereupon some of said moltensilicon in said green body reacts with some of the boron carbide in saidgreen body to produce silicon carbide.

2. A method as set forth in claim 1 wherein said granular boron carbideconsists of a variety of particle sizes,

some particles being relatively coarse and some being relatively fine.

3. A method as set forth in claim 2 wherein said heating is carried outin a vacuum and said temperature is at least about l,500C but less thanabout 1,850C.

4. A method as set forth in claim 3 wherein said temporary binder iscarbonizable, and said binder is carbonized to produce carbon in saidgreen body, and some of said molten silicon reacts with substantiallyall of said carbon to produce silicon carbide.

5. A method as set forth in claim 3 wherein finely divided carbon isincorporated in the initial mixture, and some of said molten siliconreacts with substantially all of said carbon to produce silicon carbide.

6. A method as set forth in claim 4 wherein finely divided carbon isincorporated in the initial mixture, and some of said molten siliconreacts with substantially all of the carbon present in the green body toproduce silicon carbide.

7. A method as set forth in claim 6 wherein some of said granular boroncarbide is coarser than grit, and said granular boron carbide has amaximum particle size below about 300 microns.

8. A method as set forth in claim 7 wherein said initial mixturecontains about 2 parts of carbon to 98 parts of boron carbide, and saidtemperature is about l,600C.

9. A method as set forth in claim 1 which comprises: preparing asubstantially homogeneous initial mixture of about 2 parts finelydivided carbon, about 98 parts granular boron carbide of a variety ofparticle sizes with a maximum particle size below about 300 microns andwith some being coarser than 180 grit, and a suitable quantity of acarbonizable binder; forming said initial mixture into a desired shape;setting the binder to surface of said green body and subsequentlypermeates said green body, whereupon some of said molten silicon in saidgreen body reacts with substantially all of the carbon present in saidgreen body and with some of the boron carbide in said green body toproduce silicon carbide.

2. A method as set forth in claim 1 wherein said granular boron carbideconsists of a variety of particle sizes, some particles being relativelycoarse and some being relatively fine.
 3. A method as set forth in claim2 wherein said heating is carried out in a vacuum and said temperatureis at least about 1, 500*C but less than about 1,850*C.
 4. A method asset forth in claim 3 wherein said temporary binder is carbonizable, andsaid binder is carbonized to produce carbon in said green body, and someof said molten silicon reacts with substantially all of said carbon toproduce silicon carbide.
 5. A method as set forth in claim 3 whereinfinely divided carbon is incorporated in the initial mixture, and someof said molten silicon reacts with substantially all of said carbon toproduce silicon carbide.
 6. A method as set forth in claim 4 whereinfinely divided carbon is incorporated in the initial mixture, and someof said molten silicon reacts with substantially all of the carbonpresent in the green body to produce silicon carbide.
 7. A method as setforth in claim 6 wherein some of said granular boron carbide is coarserthan 180 grit, and said granular boron carbide has a maximum particlesize below about 300 microns.
 8. A method as set forth in claim 7wherein said initial mixture contains about 2 parts of carbon to 98parts of boron carbide, and said temperature is about 1,600*C.
 9. Amethod as set forth in claim 1 which comprises: preparing asubstantially homogeneous initial mixture of about 2 parts finelydivided carbon, about 98 parts granular boron carbide of a variety ofparticle sizes with a maximum particle size below about 300 microns andwith some being coarser than 180 griT, and a suitable quantity of acarbonizable binder; forming said initial mixture into a desired shape;setting the binder to obtain a coherent green body having a specificgravity of about 1.7; carbonizing the binder to produce carbon in saidgreen body; and heating said green body in a vacuum in the presence of acontrolled amount of granular silicon to a temperature of about 1,600*C,whereby the silicon becomes molten, said granular silicon being sodisposed externally to said green body that said molten siliconsubstantially uniformly contacts the surface of said green body andsubsequently permeates said green body, whereupon some of said moltensilicon in said green body reacts with substantially all of the carbonpresent in said green body and with some of the boron carbide in saidgreen body to produce silicon carbide.